Scandium is extremely valuable as an additive for high-strength alloys and an electrode material for fuel cells. However, scandium has not yet been used widely due to the small production quantity and high cost thereof.
Incidentally, scandium is contained in nickel oxide ore such as laterite ore and limonite ore in a trace amount. However, the grade of nickel contained in the nickel oxide ore is low, and the nickel oxide ore has not been thus industrially utilized as a raw material of nickel for a long time. Hence, studies on industrially recovering scandium from the nickel oxide ore have been hardly conducted either.
In recent years, however, the HPAL process has been emerging as a practical method, in which nickel oxide ore is introduced into a pressure vessel along with sulfuric acid, and heated at a high temperature of about 240° C. to 260° C. to allow solid-liquid separation into a leachate containing nickel and a leach residue. In this HPAL process, it is possible to separate the impurities by adding a neutralizing agent to the leachate thus obtained and then to recover nickel as nickel sulfide by adding a sulfurizing agent to the resulting leachate. Thereafter, it is possible to obtain electric nickel or a nickel salt compound by subjecting the resulting nickel sulfide to a known nickel purification process.
In the case of using the HPAL process as described above, scandium contained in the nickel oxide ore is contained in the leachate along with nickel (see Patent Document 1). Thereafter, scandium is contained in the acidic solution (post-sulfuration liquid) after the addition of the sulfurizing agent while nickel is recovered as nickel sulfide as a neutralizing agent is added to the leachate obtained in the HPAL process to separate impurities and subsequently a sulfurizing agent is added to the resulting leachate, and it is thus possible to effectively separate nickel and scandium from each other by using the HPAL process.
As a method for recovering scandium from the acidic solution described above, a method for recovering scandium by adsorbing scandium to a chelating resin or the like having an iminodiacetate salt as a functional group to separate scandium from the impurities and to enrich scandium is proposed, for example, in Patent Document 2.
Patent Document 2 discloses a method for producing high purity scandium oxide from a substance containing scandium in a trace amount. Specifically, a method for producing high purity scandium oxide is disclosed, which includes a leaching step of obtaining a scandium-containing solution from an oxide containing scandium in a trace amount, a liquid adjusting step, an extraction step of forming a chelating resin which has adsorbed scandium, a washing step of washing the scandium-adsorbed chelating resin with a dilute acid, a backward extraction step of eluting the scandium-adsorbed chelating resin with a strong acid to obtain a scandium-containing solution, a precipitation step of obtaining a precipitate of scandium from the scandium-containing solution by using a precipitant, and a step of calcining the precipitate.
However, in the case of using this method described in Patent Document 2 on its own, although the distribution of iron, aluminum, chromium or the like into the eluate is significantly minor, they are contained in a larger amount as compared to the scandium contained in the raw material, and it thus takes time and labor to repeat the adsorption and elution a number of times. In addition, some impurities are distributed in the eluate to the same extent as scandium, and it is thus difficult to separate scandium from the impurities in some cases.
Further, Patent Document 3 discloses a method for recovering high purity scandium oxide from a scandium-containing solution by a solvent extraction method. Specifically, it is a method in which, first, an organic solvent prepared by diluting 2-ethylhexylsulfonic acid-mono-2-ethylhexyl with kerosene is added to a scandium-containing solution of an aqueous phase containing at least one or more kinds of iron, aluminum, calcium, yttrium, manganese, chromium, or magnesium in addition to scandium to extract the scandium component into the organic solvent. Subsequently, in order to separate yttrium, iron, manganese, chromium, magnesium, aluminum, and calcium extracted into the organic solvent along with scandium, scrubbing is performed by adding an aqueous solution of hydrochloric acid to the organic solvent to remove yttrium, iron, manganese, chromium, magnesium, aluminum, and calcium, and an aqueous solution of sodium hydroxide is then added to the resulting organic solvent to convert the scandium remaining in the organic solvent into a slurry containing Sc(OH)3. Thereafter, Sc(OH)3 obtained by filtering this slurry is dissolved with hydrochloric acid to obtain an aqueous solution of scandium chloride, oxalic acid is added to this resulting solution to form a precipitate of scandium oxalate, and the precipitate is filtered to separate iron, manganese, chromium, magnesium, aluminum, and calcium into the filtrate, and then the precipitate filtered is calcined, thereby obtaining high purity scandium oxide.
However, in the case of using the solvent extraction method, it is required to handle a large amount of solvent since scandium is contained in the nickel oxide ore in an extremely small amount and the concentration thereof is low and there are difficulties from the viewpoint of recovery rate and cost since the capacity of equipment increases.
As described above, a technique to efficiently take out and utilize scandium or scandium oxide contained in the nickel oxide ore has not been found out.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. H03-173725    Patent Document 2: Japanese Unexamined Patent Application, Publication No. H09-176756    Patent Document 3: Japanese Unexamined Patent Application, Publication No. H09-291320